Visual System: Anatomy and Physiology Flashcards
Slide Set 4 (Wk3 L2/Wk 4 L1) Topics: - Ganglion cells in Retina - LGN - Striate cortex - Extra-striate cortex - Two streams
What is spatial frequency?
cyclical changes in space (high or low)
• More luminance changes per unit length
• Something is happening more “frequently”
e. g.
high: IIIIIII
low: I I I I
What are sine wave demonstrations of spatial frequency called?
Sine wave GRATINGS
Fourier analysis
All images can be mathematically considered as a summation of sine waves of different spatial frequencies
*example: adding higher frequencies to create square-waves
What is a fourier decomposition/transformation?
tells us the low, medium, and high frequency wave components of an image
e.g. *inverse fourier transformation: removing the high frequency components to see the low frequency components
How are spatial frequencies related to our limits of acuity?
the size of our cones (S/M/L) determines whether they can be fully activated or inhibited within the spaces of a sine wave
example: when not small enough to match a high SF wave, cones are both inhibited and uninhibited, and we perceive light coming from everywhere (undifferentiated)
What is spatial frequency selectivity in ganglion cells?
neurons (ganglion cells) are selective/tuned to specific mediums of spatial frequencies; will not fire if exposed to a higher or lower SF
What is phase sensitivity/selectivity of ganglion cells?
whether the neuron fires (above/below baseline) depends on the exact position of the stimulus relative to the receptive field
*SF remains the same, just phase or location changes
What is topographic mapping?
Things that are adjacent in the world are represented by adjacent neurons in the thalamus
*also occurs in the cortex
What do the first two levels of the LGN correspond to?
Magnocellular layers
associated with signals from rods, responsible for motion and spatial location, and dorsal stream of the visual pathway
- larger cells
- receive input from M ganglion cells
What do the bottom four levels of the LGN correspond to?
Parvocellular layers
associated with signals from cones, responsible for acuity; ventral stream of visual pathway
- smaller cells
- receive input from P ganglion cells
What are in between the 6 layers of the LGN?
koniocellular layers
Which layers of the LGN are received from the same side, and which layers are received from opposite sides?
Ipsilateral eye (same-side): layers 2,3, and 5
Contralateral eye (opposite-side): layers 1,4, and 6
What are the different names used for the primary visual cortex?
- striate cortex
- V1
- primary visual cortex
What is the cortical magnification factor of V1?
foveal representations are ʻexpandedʼ relative to peripheral representations
What are the three V1 cells?
1) Simple cells:
2) Complex cells:
3) End-stop cells:
* all are SF and orientation selective
Simple Cell: orientation, phase, motion, and length sensitivity
Orientation: selective
Phase: sensitive
Motion: do not require motion
Length: not selective
Complex Cell: orientation, phase, motion, and length sensitivity
Orientation: selective
Phase: insensitive
Motion: sensitive (require motion, sometimes 1 direction)
Length: not selective
End-Stop Cell: orientation, phase, motion, and length sensitivity
Orientation: selective
Phase: insensitive
Motion: direction sensitive (require motion in 1 direction)
Length: selective (encode corner; lines STOP)
What is orientation selectivity?
depending on the orientation of the object (horizontal vs vertical) the V1 cell will or will-not fire
*unique to V1 cells; not present in retina/thalamus cells
What are the two types of simple cells?
Edge detectors: 2 subfields (+)(-)
Stripe detectors: 3 subfields (+)(-)(+)
How are complex and end-stop cells different?
End-stop cells are length-specific
- note: otherwise they are very similar: orientation specific, phase non-specific, motion specific, direction specific (although only sometimes in complex)
What are columns?
a vertical arrangement of neurons running through the striate cortex; neurons within a single column tend to have similar receptive fields and orientation preferences
- this is how V1 is organized
What are the different types of columns?
- location columns
- orientation columns
- ocular dominance columns
Location columns
- columns of neurons that all have their receptive field on roughly the same location on the retina
- run perpendicular to the cortex
Orientation columns
- column of cells that all prefer the same orientation
- aligned perpendicular to cortex (like location columns)
*different perpendicular penetrations prefer different orientations
Ocular dominance columns
a column that prefers input for either the left eye or the right eye
Hypercolumns
a block of striate cortex containing at least two sets of columns, each covering every possible orientation (0–180 degrees), with one ocular dominance column preferring each eye
simpler words: a bundle of adjacent location columns that vary in orientation and eye-selectivity
- alternate R/L
- all are encoding the same part of the world
- these columns are how V1 is organized
How is contrast defined in a stimulus?
how high or low the amplitude is
- high amplitude = high contrast
- low amplitude = low contrast
What is a Contrast Sensitivity Function (CSF)?
the threshold of contrast that is necessary to detect a stimulus; changes depending on SF
*as spatial frequency increases, contrast sensitivity must also increase
Where are the two possible streams for visual information to go after V1?
Ventral Stream: “what” system; temporal lobe
Dorsal Stream: “where/how” system; parietal lobe
What are the levels of the extra-striate cortex?
V2: illusory contours
V3: donʼt really know - possibly combining
information to construct 3D percepts
V4: color
MT (Medial Temporal): motion
IT (Inferior Temporal): shapes, form, faces
What are elaborate cells?
very specific cells that respond selectively to complex forms
*e.g. faces are extremely complex, Fusiform Face Area (FFA) is a structure in IT/temporal lobe that responds specifically to faces
